The present invention relates to a manually backdrivable ball screw assisted braking system and more particularly, to a braking system having a modulator with a powered pressure generating mechanism assisted by a manually backdrivable ball screw.
U.S. Pat. No. 5,246,283 describes an electro-hydraulic brake apply system for pressurizing a braking circuit. The braking system includes an actuator that effects pressurization of the braking system by turning a power screw through a motorized gear train. The power screw is threadably engaged with a nonrotative nut which drives a piston through reciprocable movement to control pressure within a wheel brake. The actuator provides pressure delivery functions and also functions as a pressure amplifier eliminating the need for a conventional master cylinder power booster.
To ensure that the vehicle operator experiences the brake pedal feel typically associated with braking system operation, an accumulator is provided. During normal braking operation of the system, fluid from the master cylinder flows into the accumulator which provides compliance. An accumulator shut-off valve is provided between the master cylinder and the accumulator and a normally open solenoid valve is provided between the accumulator and the actuator. The electro-hydraulic brake apply system provides normal braking, power assist, antilock braking and traction control modes of operation. The electro-hydraulic brake apply system uses one normally open solenoid valve per braking channel and also utilizes the separate accumulator for pedal feel emulation in the system. The accumulator uses a spring to provide an approximation of the conventional pedal feel associated with braking activity.